The present invention generally relates to methods and systems for deposition of materials on substrates using plasmas and the treatment of objects using microwave radiation and plasma.
Deposition technologies include physical vapor deposition (PVD), chemical vapor deposition (CVD) (either at atmospheric pressure (APCVD) or reduced pressure (LPCVD)), electroplating, evaporation, thermal flame spray, and thermal plasma spray. Many of these deposition technologies are used for the manufacture of materials layers such as semiconductors, carbon-nanotubes, industrial coatings, biomedical coatings, and the like. Oftentimes a balance has to be struck between technical concerns such as layer adhesion, contamination from undesirable elements, deposition rates, and uniformity (both on a global and on a microscopic scale), and commercial concerns such as the cost of performing such a deposition (materials costs and the effective use of the materials) as well as the cost of the manufacturing equipment deployed.
Generally, processes that employ a vacuum or reduced pressure environment are subject to higher capital equipment costs and demonstrate lower deposition rates. However, the benefit of operating in a reduced pressure environment is often a reduction of contamination and an increase in uniformity and adhesion effectiveness. Furthermore, some processes may not work at all at higher pressures and therefore require a lower pressure or vacuum level operating regime.
Plasma deposition technologies such as PVD and CVD are commonly deployed in areas such as the manufacturing of semiconductor devices. Several methods for generating plasmas are known in the art. Arc plasmas create a plasma by applying a DC voltage between two elements such as an anode and a cathode. The resulting stream of electrons (arc) is responsible for creating very high temperatures in their path through collisions with other molecules and atoms in the arc discharge region. A common problem with arc discharge plasmas is that they consume their electrodes over time. In other words, the arc sputters material from the electrodes, which is subsequently co-deposited or entered into the plasma area. In several processes such as in the deposition of materials that are required to remain very pure, such co-deposition can be detrimental, even at very low contamination levels. As an example, even small amounts of co-deposited metals can be detrimental to the functioning of semiconductors and solar photovoltaic materials.
Inductively Coupled Plasma (ICP) sources typically employ an electrical coil powered by radio frequency signal (around 1-13 MHz is common range of frequencies). The RF signal generates a rapidly changing electromagnetic field. This field can be coupled into a chamber to produce a plasma.
Electron Cyclotron Resonance or ECR plasma sources are commonly used to support deposition chemistries for various materials. ECR sources combine a microwave source (typically operated between 1 and 10 GHz) and a permanent- or electro-magnetic field, in which the microwave source supplies power to the plasma discharge region and where the magnetic field is responsible for the creation of helical paths for charged particles such as electrons and ions. Thus, because of the helical paths, the collision probability between charged particles and neutral particles is significantly increased, resulting in much longer residence times for the charged particles in the plasma region and an enhanced interaction time between the charged particles and other particles in the plasma. This enhanced residence time allows the charged particles (particularly the electrons) to create additional ionized particles in the plasma, resulting in much higher charge concentrations in the plasma region. These higher charge concentrations result in higher extraction rates of the desired particles. This is particularly useful in processes such as ion assisted deposition or in ion doping processes. Furthermore, the longer residence time of the electrons allows for an overall increase of the plasma temperature.
ECR plasmas are very common in the manufacturing of semiconductor devices. Most ECR plasma systems require vacuum levels well below atmosphere to be able to operate, and thus require expensive equipment. However ECR phenomena have been observed at elevated pressures as well.
In general, plasmas exhibit some unique characteristics such as the formation of (meta) stable surface waves in which plasma waves can be emitted over long distances away from their source of origin. Plasma sources that deliberately enhance the formation of waves are Surface Wave Plasma sources (SWPs). They are also referred to as “Surfatrons”. Surfatrons are plasma sources that are deliberately designed to create enhanced plasma wave operations.
Flame Spray Plasmas (FSPs) create a plasma flame, which is created by the chemical reaction of one or more gasses (usually the combination of a carrier gas such as methane and a reaction gas such as air or pure oxygen) while coming out of an instrument such as a torch. The material that is to be deposited is introduced into the flame, typically in powder or sometimes in solid form, whereby rapid melting of the material occurs. The molten material/plasma stream is then aimed at the substrate or surface to be coated. The plasma temperature of a FSP system is typically in the range of 2,000-5,000° C.
Thermal Spray Plasmas (TSPS) do not rely on a chemical reaction, but rather rely on physical processes to create a plasma and a molten particle stream. A typical TSP will use either a DC plasma arc (also called a DC Plasmatron) or a radio-frequency induced plasma (also called an inductively coupled Plasmatron). In either case, a plasma is created and the to-be-coated material is introduced into the plasma stream, where it is rapidly melted. The plasma/material stream is then aimed at a substrate where the material deposits and re-solidifies. The plasma temperature of a TSP system is typically in the range of 5,000-12,000° C.
The above mentioned processes for plasma generation such Inductively Coupled Plasmas, ECR plasmas, Flame Spray Plasmas, and Thermal Spray Plasmas are all commonly known in the art and have all been used or attempted to be used for the deposition of materials used in semiconductor manufacturing as well as for the deposition of photovoltaic active layers and other areas where deposition of materials is desired.
Common problems with the application of these plasmas to materials and substrates involve the co-deposition of undesirable materials that are introduced through either the erosion of the chamber that contains the plasma, or by the use of starting materials that already contain such contamination. The very high temperature of plasmas essentially evaporates some material of the chamber and electrodes surrounding it. Strategies such as the use of shielding or liners, or the use of chamber materials that are not contaminating when co-deposited are a common practice. One disadvantage of shielding and liners is that they too eventually get coated with the to-be-deposited materials or with other residual process effluents. Such depositions ultimately may result in the materials flaking off and falling on the substrate that is in process. These unintended particles or flakes are generally very destructive to the semiconductor devices in process, and great care is typically taking to minimize any risk of particles falling on the substrate. Oftentimes monitoring and periodic cleaning processes are employed to ensure that the flaking of materials is very limited or prevented as much as possible.
Low pressure plasmas have a tendency to also have low deposition rates. Low deposition rates can mean long process times, which means low equipment throughput. The cost of vacuum equipment is typically very high and the combination of long deposition times and high equipment cost is usually not desirable. However, vacuum based processes typically exhibit good adhesion to the substrate because of the absence or lowering of surface contamination (water molecules are a primary culprit in poor adhesion). Furthermore, vacuum chambers typically allow for the creation of stable, large area plasmas which allow for good uniformity across a substrate. Uniformity of the deposited layer is important because the performance of the layer is oftentimes critically dependent on the layer thickness. Uniformities where the layer thickness varies less than a few percent of the overall layer thickness across the substrate are often the goal. Various strategies have been employed to ensure layer uniformity, oftentimes involving moving either the deposition source or the substrate in a pattern across a target area. Other strategies involve the design of the source and gas injection system in such a way that diffusion of the deposition occurs over a large, uniform area.